1. Field of the Invention
The present invention relates generally to an optical waveguide fiber designed to compensate total dispersion, and particularly to an optical waveguide fiber designed to compensate total dispersion substantially equally over a range of wavelengths.
2. Technical Background
Dispersion compensation techniques in telecommunications systems or links have been used successfully. A technique useful in links already installed is one in which total dispersion (also called chromatic dispersion) is compensated by an appropriately designed waveguide fiber formed into a module that can be inserted into the link at an access point such as an end of the link. A drawback of this technique is that the compensation module adds loss to the system without adding useful system length. For situations in which the system loss budget has a small margin, the addition of a compensation module can cause unacceptably low signal to noise ratio.
Another dispersion compensation scheme involves the use of both positive and negative dispersion fibers in the cables of the link. Each cable can contain both positive and negative total dispersion waveguide fibers, or the link can be formed using cables having only positive dispersion together with cables having only negative dispersion. This compensation scheme avoids the drawback associated with the compensation module but necessarily complicates the installation and maintenance of the system. That is, the dispersion sign of a particular cable or of the fibers in the cable must be identified during installation. Also, an inventory of replacement cables would be increased over that required for standard systems because dispersion sign is an additional variable that must be taken into account in maintaining an effective inventory.
More recently, an alternative dispersion compensation technique has been developed in conjunction with a particular optical waveguide fiber having a total dispersion and a total dispersion slope which effectively mirrors that of the transmission fiber. That is, the ratio of total dispersion to total dispersion slope, xcexa, has the same value for the transmission fiber and for the compensating fiber. This fiber type is disclosed and discussed in U.S. provisional application S.No. 60/217,967, incorporated herein by reference in its entirety.
For the telecommunications system in which mirror fiber is used, the compensation is said to be perfect in that the end to end accumulated dispersion of a span including a transmission fiber and a compensating fiber is zero across the wavelength range of operation. The result of such a configuration is that signals in the fiber traverse significant span lengths in which the total dispersion is zero or near zero.
However, in certain applications it may be desirable to use the 1:1 length ratio of transmission to dispersion compensating optical waveguide fiber, as in the case of certain mirror fiber, but still maintain a non-zero local dispersion to avoid dispersion penalties due to four wave mixing and cross phase modulation. In this case, one would need a compensating waveguide fiber that mirrored the total dispersion slope but not the total dispersion of the transmission fiber.
In addition, perhaps because of consideration of the effective area or attenuation of the compensating fiber, one may wish to use a length ratio other than 1:1, for example a ratio of 1.5:1, or 2:1, where the longer length is typically taken to be the that of the transmission fiber.
There is therefore a need for dispersion compensating optical waveguide fibers designed to meet a variety of compensation formats that derive from the variety of system performance requirements together with a desired transmission to compensating fiber length ratio.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments.
The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion, sometimes called chromatic dispersion, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows.
Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
xe2x80x83Aeff=2xcfx80(∫E2rdr)2/(∫E4rdr),
where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region. In those cases in which the refractive index of a segment is less than the average refractive index of the cladding region, the relative index percent is negative and is calculated at the point at which the relative index is most negative unless otherwise specified.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b)%, where b is radius, which follows the equation,
xcex94(b)%=xcex94(bo)(1xe2x88x92[bxe2x88x92bo/(b1xe2x88x92bo)]xcex1),
where bo is the point at which xcex94(b)% is maximum, b1 is the point at which xcex94(b)% is zero, and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
A waveguide fiber telecommunications link, or simply a link, is made up of a transmitter of light signals, a receiver of light signals, and a length of waveguide fiber having respective ends optically coupled to the transmitter and receiver to propagate light signals therebetween. The length of waveguide fiber can be made up of a plurality of shorter lengths that are spliced or connected together in end to end series arrangement. A link can include additional optical components such as optical amplifiers, optical attenuators, optical switches, optical filters, or multiplexing or demultiplexing devices. One may denote a group of inter-connected links as a telecommunications system.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending, typically expressed in units of dB, is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins. The test pertains to macro-bend resistance of the waveguide fiber.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation, typically express in units of dB/m, is measured. This increase in attenuation is the lateral load attenuation of the waveguide.
A further bend test referenced herein is the mandrel wrap test. The attenuation induced by wrapping an optical waveguide fiber about a mandrel is measured. Standard test conditions include 100 turns of waveguide fiber around a 75 mm diameter mandrel and 1 turn of waveguide fiber around a 32 mm diameter mandrel. Other mandrel sizes such as 50 mm diameter or 20 mm diameter can be used. The attenuation induced by the wrapping about the mandrel is typically expressed in dB.
A first aspect of the present invention is a dispersion compensating optical waveguide fiber having, at 1550 nm, a total dispersion in the range from xe2x88x9212 ps/nm-km to xe2x88x9235 ps/nm-km, a total dispersion slope in a range from xe2x88x920.04 ps/nm2-km to xe2x88x920.11 ps/nm2-km, and a polarization mode dispersion less than 0.10 ps/kmxc2xd. Preferably, the polarization mode dispersion is less than 0.05 ps/kmxc2xd, and more preferably less than 0.01 ps/kmxc2xd. Attenuation at 1550 nm is less than 0.25 dB/km, preferably less than 0.23 dB/km, and more preferably less than 0.22 dB/km.
In an embodiment of this first aspect of the invention, the ratio of total dispersion to total dispersion slope, xcexa, has a range from 225 to 375.
A second aspect of the invention is a dispersion compensating optical waveguide fiber having a core region which includes a central segment having an outer radius ro, and an annular segment surrounding the central segment having outer radius r1. The dispersion compensating fiber has a total dispersion in a range from xe2x88x9212 ps/nm-km to xe2x88x9235 ps/nm-km, a total dispersion slope in a range from xe2x88x920.04 ps/nm2-km to xe2x88x920.11 ps/nm2-km. The ratio of the outer radius of the central segment, ro, to the outer radius, r1, of the first annular segment is defined by the expression, 0.4 less than ro/r1xe2x89xa60.6.
In an embodiment of a waveguide fiber in accord with this aspect of the invention, the attenuation and polarization mode dispersion are the same as those set forth above in the first aspect of the invention.
In a further embodiment of this second aspect of the invention, the preferred range for the ratio of the radii is defined by the expression, 0.4 less than ro/r1xe2x89xa60.55.
A third aspect of the invention is a dispersion compensating optical waveguide fiber having a core region surrounded by a clad layer, the core region including a central segment and three annular segments successively surrounding the central segment. The term surrounding means that the successive segments of the core region are in contact with their nearest neighbors. For example, the first annular segment is abutted by the central segment at its inner surface and by the second annular segment at its outer surface. Each segment is characterized by an inner and an outer radius, as defined in the drawings and the detailed description thereof, a refractive index profile, and a relative index percent. As is stated in the Definitions section, the relative index percent represents the maximum magnitude of relative index of the particular segment unless otherwise specified. The configuration of the segments provides, at a wavelength of 1550 nm, a total dispersion in a range from xe2x88x9212 ps/nm-km to xe2x88x9235 ps/nm-km, a total dispersion slope in a range from xe2x88x920.04 ps/nm2-km to xe2x88x920.11 ps/nm2-km. The third annular segment is further characterized by a width, w3, and a center radius, rc. The relationship among the parameters, w3, rc, and the outer radius of the first annular segment r1 is, rc xe2x88x92w3/2 greater than r1+0.5 xcexcm. In a preferred embodiment, the relationship among these parameters is, rcxe2x88x92w3/2 greater than r1+1.0 xcexcm. The placement and extent of the maximum index of the third annular segment are key parameters in providing the desired properties of the optical waveguide fiber in accord with the invention.
A fourth aspect of the invention is a dispersion compensating optical waveguide fiber having a core region profile and a clad layer profile selected to provide, at a wavelength of 1550 nm, a total dispersion in a range from xe2x88x9212 ps/nm-km to xe2x88x9235 ps/nm-km, a total dispersion slope in a range from xe2x88x920.04 ps/nm2-km to xe2x88x920.11 ps/nm2-km, and, resistance to bend loss is characterized by: an induced attenuation of less than 0.05 dB at 1310 nm and less than 0.10 dB at 1550 nm when the fiber is wrapped 100 turns about a 50 mm diameter mandrel; an induced attenuation of less that 0.50 dB at 1550 nm when the fiber is wrapped 1 turn about a 32 mm diameter mandrel; an induced attenuation of less than 0.50 dB at 1625 nm when the fiber is wrapped 100 turns about a 75 mm mandrel; an induced attenuation of less than 1.0 dB/m under lateral load testing; and, an induced attenuation of less than 8 dB when the fiber is configured in a pin array bend test.
In embodiments of the optical waveguide fiber in accord with this aspect of the invention, induced attenuation in the pin array bend test is less than 7 dB and preferably less than 4 dB. The induced attenuation under lateral load bending is preferably less than 0.75 dB/m.
A fifth aspect of the invention is a dispersion compensating optical waveguide fiber having, at a wavelength of 1550 nm, a total dispersion in a range from xe2x88x9212 ps/nm-km to xe2x88x9235 ps/nm-km, a total dispersion slope in a range from xe2x88x920.04 ps/nm2-km to xe2x88x920.11 ps/nm2-km, and an effective area at 1550 nm not less than 23 xcexcm2. In a preferred embodiment of this aspect of the invention, the effective area is not less than 25 xcexcm2. More preferably, the effective area is not less than a value in the range from 28 xcexcm2 to 30 xcexcm2.
In an embodiment in accord with any one of the five aspects of the invention, the core region includes a central segment having a relative refractive index percent, xcex94o%, in the range from 0.8% to 1.7%, an inner radius zero and an outer radius, ro, in the range from 2.4 xcexcm to 3.2 xcexcm, a first annular segment, surrounding the central segment, having a relative refractive index percent, xcex941%, in the range from xe2x88x920.28% to xe2x88x920.45%, an inner radius ro and an outer radius, r1, in the range from 5.0 xcexcm to 6.7 xcexcm, and, a second annular segment, surrounding the first annular segment, having a relative refractive index percent, xcex943%, in the range from 0.235% to 0.55%, an outer radius, r3, in the range from 7.5 xcexcm to 11.0 xcexcm, center radius, rc, in the range from 7.0 xcexcm to 9.6 xcexcm, and width, w3, in the range from 0.8 xcexcm to 3.0 xcexcm. In addition, this embodiment can further include a third annular segment, surrounding the first annular segment, having a relative refractive index percent, xcex942%, in the range from zero to 0.15%, an inner radius r1 and an outer radius, r2, in the range from 5.9 xcexcm to 8.0 xcexcm. A preferred configuration of this embodiment is one in which the central segment is an xcex1-profile and xcex1 has a range from 0.8 to 3.5.
It will be understood that in each of the five aspects of the invention set forth in the summary immediately above the effective area is greater than 23 xcexcm2, preferably greater than 25 xcexcm2, and more preferably greater than a value in the range from 28 xcexcm2 to 30 xcexcm2. Also, induced attenuation under bending is as disclosed in the fourth aspect of the invention. Attenuation at 1550 nm is less than 0.25 dB/km and preferably less than 0.22 dB/km and polarization mode dispersion is less than 1.0 ps/kmxc2xd, preferably less than 0.05 ps/kmxc2xd, and more preferably less than 0.01 ps/kmxc2xd. The ratio xcexa is in the range from 225 nm to 375 nm.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.